Add service url configuration to the list

Not sure if this is intentionally left undocumented, but I found this very useful when working with a CAS server that enabled the single logout functionality.  The issue was that on logout the CAS server would post to the service url.  If we had a bunch of potential urls it made our route configuration very challenging.  While implementing something to funnel all authentications through a single action, I discovered this config param.  This allowed us to use session to store the redirect path and route all CAS authentications through a single 'service'

I'm not sure if this is an intended use or not, but since the configuration parameter existed I thought adding it to the readme might be helpful. Sorry for the two commits -- please squash.